Positive grid design has a significant impact on the performance and the life of a lead-acid battery. Positive grids are subject to severe corrosion processes in the functioning of a lead-acid battery as it fulfills the role of the active material load-bearer and active material current collector. The rate of positive grid corrosion can become the life-limiting factor of the cell or the battery. Positive grids are designed so that the internal wire members provide sufficient strength and support to contain the active material and collect the current (energy) from the bulk of the active material and transmit to the straps and the terminals via the grid lugs. The design of the positive grids and the internal wire configurations are usually determined by the alloy composition, method of grid fabrication and the type of battery service in terms of energy, power density, service type, and service life requirements.
Over the years, considerable attention has been directed to a variety of different grid designs stated to be designed to enhance various performance characteristics. As one example, U.S. Pat. No. 3,989,539 to Grabb discloses a battery grid having a plurality of interconnected vertical and horizontal grid wires surrounded by frame bars. A series of the vertical grid wires are each tapered and increase in cross-sectional area in a direction towards the top of the grid, the point of current collection to the straps. Additional vertical grid wires are positioned between adjacent tapered vertical grid wires where the additional vertical grid wires extend downwardly from the top frame wire a distance of approximately 30 to 50% of the length of a side frame wire. This grid wire design, it is stated, was developed to increase and accommodate the increased flow of electrical current to the lug of the battery plate during high current discharge rates of the battery.
U.S. Pat. No. 4,099,309 to Bender discloses a process for manufacturing a grid which comprises cutting a series of equal length, parallel slits in a blank so that an area of double width appears besides each primary slit. Utilizing the procedures of the invention, it is stated that the grids cannot only be manufactured at low cost and with a saving of material, but that any damage to the crystalline structure of the material of the mesh bars is avoided and the discharge of the voltage potential established can take place via the shortest route and by way of optimal mesh bar cross-sections.
U.S. Pat. No. 4,221,852 to Qureshi discloses a battery grid having a lug on the upper edge and spaced from the corner of the frame, a set of wires parallel to one another connecting the side edges of the frame and a set of radial arms diverging away from the upper edge to connect that upper edge to each of the other three edges of the frame. Such grids were found to provide adequate rigidity without rendering such grids unduly heavy (the typical weight said to be about 58 grams). In addition, it is stated that these grids, in accordance with the invention and having the configuration illustrated in FIG. 2, were found to have an effective resistance of 1.03 mV/A or less, representing a very marked improvement in performance over prior known grid designs.
U.S. Pat. No. 4,555,459 to Anderson et al. discloses a lightweight battery grid having an array of grid wires arranged to define a plurality of parallelograms of substantially equal size. After having stated, by way of background, that consideration in the design of a grid structure must be given to minimizing grid weight, ensuring structural integrity and providing a geometry suitable for holding the active material in an efficient manner, one of the stated objects is to provide a lightweight battery grid which efficiently serves as a conductor for the active material which is pasted onto the grid.
U.S. Pat. No. 5,308,719 to Mrotek et al. discloses a lead-acid battery grid which includes a central plate lug with the grid being constructed in such a manner that the amount of metal is concentrated in the vicinity of the lug. It is stated that substantial manufacturing advantages result from the use of such grids, and the performance of the battery can be increased to the point that individual battery plates may be eliminated, resulting in lighter weight batteries and material costs savings.
As is apparent from these prior patents, there are a variety of demanding criteria which need to be addressed to provide grid designs for lead-acid batteries. An additional complication arises because lead-acid cell and battery manufacturers have a wide variety of processes that may be used to make grids for various applications, ranging from standby power or telecommunications applications to motive power uses and to starting, lighting and ignition (SLI) automotive and truck batteries. Traditionally, such lead-acid battery grids have been made by gravity casting techniques to which attention must be given to a variety of conditions to provide defect-free cast grids. Additionally, and more recently, there are many processes which have been proposed to allow grids to be made in a continuous fashion, utilizing continuous expanded grid fabrication processes or by, for example, a direct continuous grid casting process.
U.S. Pat. No. 5,434,025 to Rao et al. describe a particularly desirable expanded grid mesh made from a directly cast strip. While such grids and the batteries utilizing such grids have highly desirable performance characteristics, it would be desirable to even further enhance the design of these grids. Thus, because such grids lack vertical frame bars, such grids can be prone to higher grid growth rates in the vertical direction in service. Indeed, this higher vertical grid growth can result in premature battery failure due to the formation of internal shorts under the negative strap, particularly under high ambient temperature conditions in battery service.
In addition, the type of battery or cell design can provide further complications. As one example, the rate of positive grid corrosion can be, and usually is, much higher in sealed lead-acid cells and batteries (often termed "VRLA" for valve-regulated, lead-acid cells and batteries) than is the case with conventional, flooded electrolyte lead-acid batteries, such as are used for SLI automotive applications. Additionally, the positive grids used in large capacity industrial battery applications are relatively very thick in comparison to automotive grids and present unique challenges in grid casting. These grids have to be cast at relatively slower speeds to minimize grid wire cracking, and such grids exhibit great susceptibility to the formation of hot tears and hot cracks at the wire intersections when many types of alloys are used.
Despite all of the considerable effort in designing grids for lead-acid cells and batteries, there still exists a need for an optimized grid design which can satisfy to a greater extent than prior efforts have achieved the many, diverse, and demanding criteria. Indeed, and despite all of this prior effort, what appears to have been overlooked is an understanding that positive grid designs have special requirements that dictate design considerations quite separate from the considerations used designing negative grids in lead-acid cells and batteries.
It is, accordingly, an object of the present invention to provide a positive grid design capable of imparting to a lead-acid battery improved electrical performance and/or an enhanced cost-benefit relationship regarding such electrical performance.
Another object of this invention is to provide a lead-acid battery having enhanced power and energy densities. A related and more specific object is to provide such batteries having improved positive active material conductivity with this grid design. A still further object lies in the provision of a positive grid design capable of achieving savings in grid material weight.
An additional object of the present invention is to provide lead-acid batteries having enhanced positive grid corrosion resistance and reduction in the vertical grid growth in the expanded positive grids.
Still another object of this invention is to provide a lead-acid battery having enhanced formation efficiency.
Yet another object of the present invention lies in the provision of lead-acid batteries having increased efficiencies in positive active material utilization.
Other objects and advantages of the present invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawings.